Abstract
Use of fiber Bragg gratings (FBGs) to monitor high temperature (HT) applications is of great interest to the research community. Standard commercial FBGs can operate up to 600 C. For applications beyond that value, specific processing of the FBGs must be adopted to allow the grating not to deteriorate. The most common technique used to process FBGs for HT applications is the regeneration procedure (RP), which typically extends their use up to 1000 C. RP involves a long-term annealing of the FBGs, to be done at a temperature ranging from 550 to 950 C. As at that temperature, the original coating of the FBGs would burn out, they shall stay uncoated, and their brittleness is a serious concern to deal with. Depositing a metal coating on the FBGs prior to process them for RP offers an effective solution to provide them with the necessary mechanical strengthening. In this paper, a procedure to provide the FBG with a bimetallic coating made by copper and nickel electrodeposition (ED) is proposed, discussing issues related to the coating morphology, adherence to the fiber, and effects on the grating spectral response. To define the processing parameters of the proposed procedure, production tests were performed on dummy samples which were used for destructive SEM–EDS analysis. As a critical step, the proposed procedure was shown to necessitate a heat treatment after the nickel ED, to remove the absorbed hydrogen. The spectral response of the FBG samples was monitored along the various steps of the proposed procedure and, as a final proof test for adherence stability of the bimetallic coating, along a heating/cooling cycle from room temperature to 1010 C. The results suggest that, given the emergence of Kirkendall voids at the copper–nickel interface, occurring at the highest temperatures (700–1010 C), the bimetallic layer could be employed as FBG coating up to 700 C.
Highlights
Fiber Bragg gratings (FBGs) are fiber optic sensors whose operating principle is based on theBragg diffraction effect [1]
A procedure to provide the FBG with a bimetallic coating made by copper and nickel electrodeposition (ED) is proposed, discussing issues related to the coating morphology, adherence to the fiber, and effects on the grating spectral response
Production of FBGs is done by various techniques, the most common for commercial production being the irradiation of the fiber segment by ultraviolet (UV) light with a periodically modulated intensity
Summary
Fiber Bragg gratings (FBGs) are fiber optic sensors whose operating principle is based on theBragg diffraction effect [1]. Fiber Bragg gratings (FBGs) are fiber optic sensors whose operating principle is based on the. Along a short segment (typical 10 mm) of the optical fiber, a diffraction grating is written to produce a periodic modulation of the refraction index of the core of the fiber itself. The UV light modifies the refraction index of the core of the fiber, somehow to its modulated intensity pattern, producing the diffraction grating. FBGs are subjected to thermal instability with a fast decay of their features. Such decay is triggered with no recovery even by a temporary reaching of HT, this could occur with a long exposure at lower (than 600 ◦ C) temperature. It was observed that starting from 600 to 700 ◦ C, the features of FBG decay until complete erasure of the FBG [2,3,4,5,6]
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